1. Field of the Invention
The present invention relates to a variable valve system, and an engine system and a vehicle including the same.
2. Description of the Background Art
Conventionally, many kinds of variable valve timing (VVT) mechanisms that control the opening/closing timing of an intake valve or an exhaust valve have been developed in order to improve fuel consumption, reduce toxic substances in exhaust gas, and achieve high power output in a particular revolution range.
Some of those variable valve timing mechanisms use an actuator such as a hydraulic cylinder and an electric motor, for example. However, these actuators are expensive and the use of such an actuator increases the size of the variable valve timing mechanism.
In general, the space occupied by an engine in a motorcycle is smaller than that of a four-wheeled automobile and the like. There has been a demand for motorcycles that can be manufactured less costly. Therefore, there is a demand for more inexpensive and compact variable valve timing mechanisms for use in motorcycles. Thus, it was difficult to use the variable valve timing mechanisms including actuators as described above in motorcycles.
A rotation phase generator has been suggested as a variable valve timing mechanism that can be made compact (see JP 9-324614 A).
In the rotation phase generator, an input member including two intermediate members is rotated together with the revolution of the engine. When centrifugal force acting on a weight portion of the two intermediate members is greater than the energizing force of a coil spring connecting these intermediate members, the rotation phases of the input member and an output member connected to a camshaft change, so that the valve timing changes.
With the rotation phase generator, the valve timing is controlled depending on the mechanical arrangement, and therefore the cost and size can be reduced.
In the above-described variable valve timing mechanism, an ECU (Electronic Control Unit) does not switch the valve timing but the valve timing is mechanically switched at a prescribed revolution speed of the engine by centrifugal force generated by the revolution of the engine. The ECU determines the valve timing based on the revolution speed of the engine and controls the fuel injection amount, the fuel injection timing, and the spark ignition timing. In practice, however, the revolution speed of the engine at which the valve timing switches varies.
In this case, the ECU cannot accurately determine the actual valve timing during driving. Therefore, control of the fuel injection amount, the fuel injection timing, and the spark ignition timing carried out by the ECU could be inconsistent with actual valve timing in some cases. This gives rise to problems such as increase in toxic substances in exhaust gas.